1. Field
The present disclosure relates to a DC power supply equipment that uses a plurality of isolated DC-DC converter units and supplies a load with a DC voltage.
2. Description of Related Art
FIG. 10 is a circuit diagram of conventional DC power supply equipment. The DC power supply equipment as shown in FIG. 10 comprises capacitors 9a, 9b, and 9c connected in series between the negative and positive terminals of a DC power supply 50. A high frequency inverter 31a for converting a DC voltage into a high frequency AC voltage is connected between the both terminals of the capacitor 9a, and a rectifier circuit 33a is connected through a transformer 32a to the output side of the high frequency inverter 31a. In parallel to the series circuit consisting of the high frequency inverter 31a, the transformer 32a, and the rectifier circuit 33a, a series circuit consisting of a high frequency inverter 41a, a transformer 42a, and a rectifier circuit 43a is connected. An isolated DC-DC converter unit A (also referred to simply as “unit” A) is composed of the capacitor 9a, the inverters 31a, 41a, the transformers 32a, 42a, and the rectifier circuits 33a, 43a. 
Similarly to the unit A, a unit B is composed of the capacitor 9b, the inverters 31b, 41b, the transformers 32b, 42b, and the rectifier circuits 33b, 43b; and a unit C is composed of the capacitor 9c, the inverters 31c, 41c, the transformers 32c, 42c, and the rectifier circuits 33c, 43c. The positive side output terminals of all the units A, B, and C are connected together, and the negative side output terminals of all the units are connected together to supply a DC voltage to a load not shown in FIG. 10.
In the unit A, for example, having a construction described above, the inverter 31a converts once a DC voltage into a high frequency AC voltage, the transformer 32a isolates and transforms the AC voltage, and then the rectifier circuit 33a rectifies the AC voltage to convert again into a DC voltage. Thus, functions of power supply equipment, voltage conversion and isolation from the load, are performed. A high frequency is employed because it generally allows a smaller transformer size. The series circuit composed of the inverter 31a, the transformer 32a, and the rectifier circuit 33a in the unit A, and the series circuit composed of the inverter 41a, the transformer 42a, and the rectifier circuit 43a in the unit A are called as an isolated DC-DC converter and a technology known in the art. Similar situations are applicable to the units B and C.
The DC input sides of the units A, B, and C are connected in series because of the following reason. The withstand voltages of the semiconductor switching devices used in the power conversion circuits such as the inverters 31a, 31b, and 31c have to be higher than the voltages Ea, Eb, and Ec of the capacitors 9a, 9b, and 9c. On the other hand, semiconductor switching devices generally tend to exhibit poor high speed switching performance with a withstand voltage higher than a certain value. The withstand voltage of semiconductor switching devices that are capable of operating at a switching frequency of around several tens of kHz is about 1,200 volts at the maximum, at present.
Semiconductor switching devices exist that exhibit a withstand voltage of 3,300 volts, which is higher than the voltage of the DC power supply 50, for example 2,000 volts. However, they can operate practically at a switching frequency of 1 kHz at the highest. One unit can be constructed using such semiconductor switching devices in the inverter 31a and connected to the DC power supply 50. However, the low switching frequency of the semiconductor switching device disturbs down-sizing of the transformer.
Accordingly, in the past, the DC input side of a plurality of units, the three units A, B, and C in the power supply equipment in FIG. 10, are connected in series to lower the DC input voltage per one unit and allow the use of the semiconductor switching device having a withstand voltage not higher than 1,200 volts. Because the rectifier circuit 33a in the unit A, for example, is isolated from the input side, the output potential can be determined at an arbitrary value. Thus, the power capacity of the power supply equipment can be achieved by connecting in parallel with rectifier circuit 33b and 33c of the unit B and the unit C. Although three units are connected in series in the DC input side of the power supply equipment of FIG. 10, the number of series connection of units can be determined arbitrarily in consideration of the voltage of the DC power supply 50 and the withstand voltages of the semiconductor switching devices and other components.
Japanese Unexamined Patent Application Publication No. 2014-018028 (paragraphs [0014] to [0022] and FIG. 1, in particular) (“JP '028” hereinafter) discloses a conventional technology for achieving power conversion equipment with a high input voltage using semiconductor switching devices with a relatively low withstand voltage by series connection of circuits. In this conventional technology, a plurality of chopper cells are connected in series. A control section compares voltage threshold values, the number of which is equal to the number of series connection of the chopper cells, with an output voltage command value of the power conversion equipment. Based on the comparison result, the semiconductor switching devices of the chopper cells corresponding to each voltage threshold value are one-pulse-operated to equalize the capacitor voltages in each chopper cell.
Another conventional technology is known in which conversion circuits such as chopper cells are not series-connected but switching devices themselves are connected in series. This technology, however, needs to strictly adjust switching timings in order to balance the voltages applied to the switching devices in the switching process. Thus, operation at high switching frequencies is difficult, which confines practical application examples.
In addition, the conventional technology shown in FIG. 10 also needs equal output power of the units. If there is imbalance in the output power between the units, the input voltage of the unit with a high output power decreases while the input voltage of the unit with a low output power increases. If operation is continued in this situation, the semiconductor switching devices used in a unit with an increased input voltage may undergo an overvoltage higher than the withstand voltage, causing failure of the power supply equipment.
In the DC power supply equipment of FIG. 10, a unit, for example unit A, has two parallel-connected isolated DC-DC converters, one isolated DC-DC converter composed of the inverter 31a, the transformer 32a, and the rectifier circuit 33a, and another isolated DC-DC converter composed of the inverter 41a, the transformer 42a, and the rectifier circuit 43a. The reason for this construction is described below.
From the requirement for energy saving in recent years, DC power supply equipment is demanded high efficiency not only around a rated power but also in light load conditions. Power conversion circuits used in DC power supply equipment usually operate at input and output voltages in a predetermined range. Thus, approximately proportional relationship exists between the power and running current in the circuit. When the current is small in a light load condition, resistive losses in semiconductor devices such as semiconductor switching devices and free-wheeling diodes, and in the winding of a transformer decreases. On the other hand, the iron loss in a transformer, which depends on the voltage but little depends on the current, is so-called a fixed loss, which does not change in the light load condition. Thus, a predetermined efficiency is hardly maintained unless the losses decrease when treating a lower power under the light load condition.
Accordingly, the DC power supply equipment of FIG. 10 can improve efficiency thereof in which a plurality of power conversion circuits, the isolated DC-DC converters in each unit, are parallel-connected in the DC input side, and one of the parallel-connected power conversion circuits is stopped operation in a light load condition to decrease the fixed loss. This type of method for improving efficiency in the light load condition has been proposed, in which a plurality of power conversion circuits are connected in parallel and the number of operation is controlled. In this method, the proportion of power capacities divided by the plural power conversion circuits can be not necessarily equal.
Japanese Unexamined Patent Application Publication No. 2006-333625 (paragraphs [0018] to [0025], and FIG. 1 and FIG. 2, in particular) (“JP '625” hereinafter) discloses an operation method in which a plurality of low capacity inverters and a plurality of high capacity inverters are all connected in parallel in the input side and in the output side, and the number of operating inverters is determined corresponding to the output power of the DC power supply equipment by selecting from the low capacity inverters, from the high capacity inverters, or from all the inverters.
In order to utilize semiconductor switching devices capable of high frequency switching operation in the conventional technology of FIG. 10, a DC input voltage is applied to a series circuit of three units to divide the input voltage to each unit into one third of the total DC input voltage, and each unit is divided into two isolated DC-DC converters connected in parallel to improve efficiency in a light load condition. Thus, the overall DC power supply equipment is constructed by three division times two division equals six divisions of isolated DC-DC converters.
This construction of multiple of isolated DC-DC converters does not occupy a huge volume because each isolated DC-DC converter has a low current carrying capacity. However, so-called dead space increases corresponding to the number of isolated DC-DC converters. In addition, the increased number of converters increases detectors and control circuits for detection and controlling functions, signal transmission components, and wiring lines in proportion to the number of converters, which causes increased costs. Therefore, the DC power supply equipment of FIG. 10 can still afford to improve in an overall size and costs.
The conventional technologies disclosed in JP '028 and JP '625 do not intend to solve the problems of size and cost increase due to increased number of chopper cells and high and low capacity inverters. The conventional technology disclosed in JP '028 is a power conversion equipment for delivering three-phase AC voltage in parallel connection of arms for three phases, an arm for one phase comprising a plurality of chopper cells connected in series. The conventional technology disclosed in JP '625 is an AC power supply system composed of a plurality of inverters connected in parallel. Thus, there has been no conventional technology that reduces the fixed loss without employing the redundancy of the isolated DC-DC converters like the construction of FIG. 10 in a DC power supply equipment comprising a plurality of isolated DC-DC converters with DC input sides thereof connected in series to take a divided portion of the DC input voltage.